The present invention relates generally to improving the thermal efficiency of heaters or furnaces and, more particularly, to modulating or controlling combustion air flow in relation to the fuel-gas flow in a gas-fired combustion system.
Gas-fired heating systems such as furnace or make-up air systems, or space heaters, typically take in outside air, heat it to a temperature set by a thermostat, and discharge the heated air inside the building. Contemporary heating systems, for example, furnaces for space heating or make-up air heating, are generally equipped with fuel valves which can be used to modulate the fuel-input rate to the heater in order to maintain a stable and controlled temperature. This type of system generally has a limited range of fuel-input modulation. In addition, these contemporary heaters suffer from a loss of thermal efficiency as the fuel-input rate is reduced below their full fuel-input rate.
Alternatively, such controlled temperature systems may also include some type of damper to open or close a combustion air by-pass. As a result of the ability to vary the amount of combustion air accessible to the system, these systems offer a broader possible range of fuel input modulation. Typically, however, the damper has only a few static positions available, which limits the actual control over the ratio of fuel to air.
Other known heating systems provide improved thermal efficiency by controlling the supply of fuel and combustion air in predetermined incremented amounts. However, these systems are complex and costly, requiring accurate sensor systems, flow control devices such as mechanical jackshafts, and the application of algorithms and control units to regulate the thermal efficiency.
Conventional modulating gas-fired burners modulate the fuel-gas flow in response to the flow rate of combustion air. Such systems are conventionally used in conjunction with power burners, which utilize so-called “drum and tube” type heat exchangers as known to those skilled in the art, and are not very amenable for use with tubular, clamshell, serpentine or other heat exchangers. In addition, the modulation of the fuel-gas flow in these conventional systems requires some type of sensor to monitor the combustion air flow rate and a valve to adjust the gas flow accordingly. In particular, a special gas valve is commonly used that responds to varying low negative pressures as measured by a pressure sensor. However, the accuracy of measuring these low negative pressures is limited and adversely affects the ability to accurately control the fuel-gas flow rate. Subsequently, the accuracy to which the air to fuel gas ratio and, subsequently, the thermal efficiency is limited in conventional modulating heating systems.
There is a need, therefore, for an efficient system and method for modulating or controlling the relative combustion air flow and fuel-gas flow of a gas-fired heating system.